TWI Industrial Member Report Summary 287/1985
R Rodwell and D A Worrall
This survey has been produced to review the methods of quality control for the ultrasonic wire bonding process, in view of the increasing criticality of the applications of the process and the increasing degree of automation of the process equipment, with consequent reduction in operator supervision.
The basic principles of the process are described, a model for the bonding mechanism discussed, and the criteria which determine bond quality are specified. In practice, quality control of ultrasonic wire bonding in production is mainly by regular batch destructive testing and by ensuring consistent performance of the bonding machine by, for example, periodic calibration. A more desirable approach is that of in-process monitoring and control of every joint made. Although in-process techniques have been extensively studied, they are currently little used because of the lack of a universal system, doubts on reliability, and access problems. The in-process monitoring and control techniques which have been studied have concentrated on detection of variations in the mechanical impedance of the bond zone, which are reflected back into the equipment's excitation system during bond formation. Active machine programming, in which bonding tool vibration amplitude is adjusted during the bonding cycle, is another approach which has led to improved joint quality. It is believed that further development of these techniques, coupled with simultaneous monitoring of associated parameters, e.g. bonding wire deformation, offers hope of improved process control.